1. Field of the Invention
The present invention relates to an ink-jet head and an ink-jet printing apparatus performing printing by ejecting an ink droplet toward a printing medium.
2. Description of the Related Art
As an ink-jet head, there is a head which instantly causes bubbling in ink by thermal energy supplied from a heater to perform printing by ejecting the ink with growth of the bubble. Such head is particularly superior in high speed printing and high density printing. In such head, the head employing a system, in which the bubble of the ink generated on the heater is communicated with the atmosphere, has been known (Japanese Patent Application Laid-open No. 10940/1992, Japanese Patent Application Laid-open No. 10941/1992, Japanese Patent Application Laid-open No. 10942/1992 and so on). The first feature of the head of this type is high ink ejection speed and high reliability. The second feature of the head of this type is that substantially all of the ink between the heater and the ejection opening can be ejected to make volumes of the ejected ink through all ejection openings substantially constant to make density fluctuation small.
According to progress of printing technology, it has been required to print smaller ink droplets with higher density. However, to make the ink droplet smaller, an ink passage becomes thinner which tends to cause lowering of ejection efficiency namely lowering of ejection speed. Therefore, problems of the reliability, such as unstability of the direction of ink ejection, unstability of ink ejection caused by increasing of the viscosity of the ink due to evaporating of the volatile component in the ink during the head resting, are caused. In this respect, the head of the type set forth above, namely the head, in which the bubble is communicated with the atmosphere, is difficult to cause the foregoing problems and can be adapted to demand for high quality printing in the future.
However, in the head of the type set forth above, the following problems are encountered. Namely, since the bubble is communicated to the atmosphere during growth of the bubble, the bubble becomes large meniscus upon communication with the atmosphere to make a re-fill time of the ink long. When next bubbling is caused without waiting for completion of re-filling, in certain case, the ink cannot form normal droplet and to cause so-called mist phenomena, in which the ink becomes a mist state, and the ink may fly in various direction to stain the printing medium.
On the other hand, conventionally, as an output means of a personal computer and so on, printers of various printing types are employed. According to speeding up of process speed of the personal computer, spreading of internet, demand for speeding up of a color image printing is increasing. Therefore, an ink-jet printer which can perform high speed printing comparable with a laser printer, can be easily adapted for color printing, and is low cost, has been widely used.
One of typical printing system of the ink-jet printers is a bubble-jet printing system which is a system heating and evaporating the ink by a thermal energy generating means and ejecting the ink droplet through the ejection opening by a pressure of the bubble generated. After ejection of the ink droplet, the vapor of the ink within the bubble is condensed to return into a liquid state to finally extinguish the bubble. While the ink in the ink passage is reduced by ejection of the ink, the ink is filled through an ink supply passage.
FIG. 15 is an explanatory illustration showing a construction of a head of a bubble-jet printing system associated with the background art. A plurality of ink passages 22 are branched from the ink supply passage 21. Thus, the ink passages 22 and the ink supply passage 21 are communicated with each other. On a tip end of each ink passage 22, an ejection opening 23 for the ink droplet is provided. In opposition to each ejection opening 23, a heater 24 (see FIG. 17) as a thermal energy generating means is provided. On the other hand, by slightly differentiating lengths of respective ink passages 22 (distance from the branching position 25 from the ink supply passage 21 to the ejection opening 23) instead of making them uniform, the positions of the ejection openings 23 are offset to permit high density printing. Since the center of the ejection opening 23 and the center of the heater 24 are located in opposition, the distance from the branching position 25 from to the ejection opening 23 is consistent with a distance (hereinafter referred to as "distance C-H") from the branching position 25 to the heater 24.
In the shown example, two hundreds fifty-six ink passages 22 are provided, in total. However, in FIG. 15, only thirty-two ink passages 22 are shown. These ink passages 22 are divided into two sets, i.e. even number passages located on the left side in the drawing and odd number passages located on the right side. In each set, the ink passages are grouped per eight into sixteen groups. The heaters 24 of eight ink passages 22 in the same group are driven simultaneously in time division so that sixteen times driving in total of heaters is set at one cycle. It should be noted that lengths of the ink passages 22 (distances from the branching position 25 to the ejection opening 23) are divided into five kinds.
Discussing this example, concerning the passages in the even number order in sequence (hereinafter referred to as "even number passages"), eight passages Seg0, 32, 64, 96, 128, . . . 224 constitute a first group. Eight passages Seg10, 42, 74, . . . 234 constitute a second group. Eight passages Seg20, 52, . . . 244 constitute a third group. Eight passages Seg30, 62, . . . 254 constitute a fourth group. Eight passages Seg8, 40, . . . 232 constitute a fifth group. Eight passages Seg18, 50, . . . 242 constitute a sixth group. Eight passages Seg28, 60, . . . 252 constitute a seventh group. Eight passages Seg6, 38, . . . 230 constitute a eighth group. Eight passages Seg16, 48, . . . 240 constitute a ninth group. Eight passages Seg26, 58, . . . 250 constitute a tenth group. Eight passages Seg4, 36, . . . 228 constitute a eleventh group. Eight passages Seg14, 46 . . . 238 constitute a twelfth group. Eight passages Seg24, 56, . . . 248 constitute a thirteenth group. Eight passages Seg2, 34, . . . 226 constitute a fourteenth group. Eight passages Seg12, 44, . . . 236 constitute a fifteenth group. Eight passages Seg22, 54, . . . 246 constitute a sixteenth group. As can be seen from the above, grouping of the ink passages are done by grouping every sixteen passages
Also, the passages in the odd number order in sequence (hereinafter referred to as "odd number passages"), similarly to the even number passages, the passages are grouped into sixteen groups, such that eight passages Seg1, 33, 65,97, 129, . . . 225 constitute a first group, eight passages Seg11, 43, 75, . . . 235 constitute a second group, eight passages Seg21, 53, . . . 245 constitute a third group, . . . eight passages Seg23, 55, . . . 247 constitute a sixteenth group. Accordingly, each group is consisted of eight even number passages and eight odd number passages and thus is consisted of sixteen passages in total.
Upon printing, the first group to the sixteenth group are driven per group in sequential order. An interval after driving one group to drive the next group is 5.9 .mu.sec.
In case of FIG. 15, the even number passages are driven to eject the ink droplet in a sequential order from the passage having short distance C-H, and the odd number passages are driven to eject the ink droplet in a sequential order from the passage having long distance C-H. The ink passages performing ejection of the ink later is influenced by the ink passages performed ink ejection earlier. Namely, the passages Seg22, 54, . . . 246 and Seg23, 55, . . . 247 of the sixteenth group is influenced by vibration of the ink passages of all groups driven in advance. Particularly, in case of the ink passage having short distance C-H, influence of vibration due to ink ejection in other group should be extended to the meniscus portion in the ejection opening portion.
FIG. 16 relates to the ink passage (the ink passage having short distance between C-H) of the sixteenth group of the even number passages of FIG. 15, and is a graph taking an elapsed time from application of the drive pulse to the first group on a horizontal axis and a position of meniscus of the ejection opening portion on a vertical axis. It should be noted that the position of the meniscus is expressed by taking the end face of the ejection opening as zero, that a positive value represents a projecting amount bulging outwardly from the ejection opening and a negative value represents an inwardly retracting amount from the ejection opening. Until the heaters of the ink passages of the sixteenth group are driven, while driving of the heaters of other groups are performed for fifteen times, the meniscus of the sixteenth group continuously expand to increase projecting amount from the end face of the ejection opening.
According to a result experimentally obtained through study by the inventors, projecting amount of the meniscus becomes greater than or equal to +3 .mu.m from the ejection opening. Then, as shown in FIG. 17, upon driving of the heater, the ink droplet 9 for printing is ejected in spherical shape, and the separated late ink droplet 9 is ejected to cause so-called broken droplet ejecting phenomenon. In this case, in comparison with other ejection openings, ink amount becomes large to make the droplet greater. For example, when so-called black solid printing is performed by ejecting ink through all ejection openings, black stripes locally having higher density can appear cyclically on the printing surface to cause degradation of the printing quality. In FIG. 17, the reference numeral 29a denotes a meniscus defined after ink ejected.
In case of the odd number passages of FIG. 15, the heaters are driven so that the ink droplets are ejected in sequential order from the ink passages having long C-H distance. The ink passage having the shortest C-H distance is present in the first group. In normal printing operation, since driving of the heaters in time division manner (sixteen times of driving of the heaters=one cycle) is repeated, the position of the meniscus of the ink in the first group becomes equivalent to that state of FIG. 16 due to influence of vibration of the ink passage caused by ejection of ink droplets from the second group to the sixteenth group and further by ejection of the ink droplets before performing ejection of the first group in the next cycle. Accordingly, since driving cycle of the heaters in time division manner is repeated, irrespective of the group belonging, the ink passage having short C-H distance can cause broken droplet ejection phenomenon.